Transparent thermoplastics have replaced glass in many applications. Exemplary products made from transparent thermoplastics include glazing for buildings, or public transportation vehicles, such as trains, buses and airplanes, lenses for eye-glasses and other optical instruments and the like. While thermoplastics are lighter and more shatter resistant than glass, their abrasion resistance is relatively low. Typically, with even ordinary use in the presence of dust, contact with abrasives, cleaning equipment, and weathering, these transparent plastics may be marred or scratched. This lack of surface hardness and abrasion resistance severely restricts the use of transparent thermoplastic materials.
There is a significant body of technology dealing with means of coating transparent thermoplastics to improve the abrasion resistance of these materials. For example, coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolysable silanes in a hydrolysis medium have been developed to impart scratch resistance. U.S. Pat. Nos. 3,708,225, 3,986,997, 3,976,497, 4,368,235, 4,324,712, 4,624,870 and 4,863,520 describe such compositions and are incorporated herein by reference in their entireties.
While these coating compositions may be useful in imparting mar resistance, they are typically rigid upon curing. Accordingly, bending or re-shaping the plastic sheets coated with these compositions often leads to micro-cracking. For this reason, current commercial hardcoatings are typically used on flat thermoplastics or pre-shaped articles. However, there is a strong need in the industry to manufacture mar-resistant articles by thermoforming pre-hardcoated thermoplastic sheets, i.e., applying a hardcoat to a plastic sheet first, then heating the sheet to its softening temperature and subsequently forcing the hot material against the contours of a mold by mechanical or pneumatic techniques. This is especially true for applications involving coating complex shapes where conventional coating processes have difficulties applying lacquer evenly to completely cover all surfaces.
Many attempts have been made in the coating industry to improve the flexibility of hardcoats. For example, U.S. Pat. No. 4,159,206 discloses a method of improving flexibility by incorporating dialkylsiloxane units through co-condensation of difunctional silane to dilute the crosslinking density of the coating composition. However, the deployment of the difunctional silane is random and cannot be easily controlled. In addition, this reference does not disclose any mechanism to relieve the stress caused by thermoforming. Accordingly, weatherability performance of the coatings disclosed in the patent may be less than desirable since stress is known to reduce coating weatherability.
U.S. Pat. No. 4,914,143 discloses improving the flexibility of the coatings by using colloidal silica having a particle size of no greater than from about 5 to about 10 nanometers. While such coating compositions may provide limited flexibility for more gentle thermoforming applications, they are still not suitable for more demanding applications, for example, those applications that require parts to be bent into smaller radius of curvature. Moreover, the employment of colloidal silica with small particle size also causes the hardcoat to be less resistant to abrasion and marring than might be desired.
U.S. Pat. No. 4,368,235 discloses coating compositions prepared by hydrolyzing an alkyltrialkoxysilane or aryltrialkoxysilane in an aqueous colloidal silica dispersion and adding to the resultant hydrolysis product a linear functionally terminated oligomeric siloxane. However, the flexibility improvement of the coatings is moderate, thus the coating composition disclosed in U.S. Pat. No. '235 may not be suitable for demanding thermoforming applications.
U.S. Pat. No. 7,482,062 discloses a silicone coating composition comprising a silicone resin which is obtained by cohydrolytic condensation of (A) an organosilicon compound, and (B) an alkoxysilane and/or (C) a fluorine-containing organosilicon compound. The '062 patent does not disclose any necessary parameters to make the disclosed coating compositions suitable for use in applications that involve thermoforming techniques.
As such, it is appreciated that there is still a need for an improved coating composition that is suitable for demanding thermoforming applications. The present invention provides an answer to that need.